Tertiary winding CT

[pwrengrds]

At what transformer size would you normally put in a CT to monitor the current on the tertiary winding of a Y-gnd/buried delta/Y-gnd transformer? I am dealing with wind farm applications, 100-150 MVA, 240kV/34.5kV sized transformers and would like to find some information on how to set them (I assume, by current rating,? but you know where that gets me), and what currents would normally be seen on the tertiary winding (I would guess low current unless you have unbalance or harmonics), I have not found much yet and would appreciate some pointers toward some books or papers.

David

 

[prc]

The maximum current that will be seen by CT is the circulating current during LG fault on HV side.Normal current will be only the equivalent third harmonic current in excitation current. It will be less than 0.5 % of full load current for this size of transformer.But first of all why a tertiary winding is required for this transformer ? I believe this is the grid transformer that is connected to the collecting transformers with Y-gnd/Delta connection.

 

[pwrengrds]

The distribution transformers are 34.5kV delta / 480 or 600 volts Yg connection. This is the transmission substation transformer which is Y gnd/Y gnd. The Y-Y TX requires a delta to stabilize the Y-Y winding's.

 

[cranky108]

Unless the tertuary windings are specified differently, they should be around 35% of the primary rating to handle the zero sequence component during a high voltage line to ground fault. Having said that the circulating current can be monitored to protect the lower current capacity of the tertuary winding.

If you are using the tertuary winding for load, be aware that in many cases the tertuary fault current can be quite large (as well as ungrounded).

 

[Marmite]

Picking up on PRC's point, if your 34.5/600V transformers downstream are all Delta star and you have no single phase 34.5kV loads then you don't necessarily need a tertiary winding. It should be possible to obtain a Yy transformer which makes use of the tank as a phantom delta winding, resulting in a cheaper transformer.
Regards
Marmite